A number of glasses having high density and exhibiting a wide transmission window in the ultraviolet range are generally known. The high density and wide transmission window in the ultraviolet range allow high stopping power and high Cerenkov light yield. Consequently, such glasses are good materials for applications requiring either fast time measurements, or energy measurements at high energy.
The following are examples of patents directed to glasses and technology related to the present invention: U.S. Pat. No. 4,717,691, Lucas et al.; U.S. Pat. No. 4,749,666; U.S. Pat. No. 5,015,281, Hall et al.; U.S. Pat. No. 5,081,076, Rapp; EP 0 234 581, Minoru et al.; EP 0 282 155, Franz et al.; and GB 2,082,168, Lucas et al.
Fluoride glasses have good optical transparency both in the infrared and ultraviolet regions of the electromagnetic spectrum. In this field, new high density fluoride glasses were developed and characterized in terms of density, ultraviolet (UV)/visible/infrared (IR) transmission, chemical stability, and glass transition/crystallization temperatures. The family of glasses found in this field have a very high density, are free of any lead or radioactive elements, and are accompanied by a low UV cut-off as well as extended transmission into the mid-IR range.
Fluoride glasses were first discovered in 1974 by Michael and Marcel Poulain, who were working with Jacque Lucas at the University of Rennes in France. These glasses were found to have good optical transparency in the UV and visible ranges. High density fluoride glasses are of particular interest because they tend to have a low UV cut-off as well as extended transmission into the mid-IR range. Possible uses for these glasses are: Cerenkov detectors, scintillation hosts, x-ray detectors, optical amplifiers, and if stable enough, laser fibers.
Other high density glasses have been reported, including a family of glasses in the BaF.sub.2 --YbF.sub.3 --ThF.sub.4 --ZnF.sub.2 system with a reported density of 6.43 g/cc. The problem with glasses in this system is that they are relatively unstable against devitrification. Samples have been cast up to a maximum thickness of approximately 3 mm, but this system contains thorium which exhibits radioactive decay. Thorium based glasses do, however, have better chemical stability and extended IR transmission compared to traditional fluorozirconate glasses.
Another high density glass family, 6.0 g/cc, was discovered by Tick in the CdF.sub.2 --Lif--AlF.sub.3 --PbF.sub.2 quaternary system, but these glasses can only be made up to a maximum thickness of 2 mm. This system also contains lead, which has been shown to have quenching affects for scintillation mechanisms in fluoride glasses.
Zhou, et al. reported glass formation in the PbF.sub.2 --BaF.sub.2 --HfF.sub.4 --ZnF.sub.2 --YbF.sub.3 system having a density range of 6.0-6.9 g/cc, a maximum thickness of 3 mm, and a UV cut-off of 254 nm. However, these glasses were found to be very susceptible to attach by moisture as they exhibited a thick reaction layer after being immersed in water for a period of only one hour. This would make the glasses unsuitable for uses in an ambient atmosphere.
In his work, Zhou was originally trying to synthesize glasses the BaF.sub.2 --YbF.sub.3 --ZnF.sub.2 --HfF.sub.4 system by substituting Hf in place of the Th found in the BaF.sub.2 --YbF.sub.3 --ZnF.sub.2 --ThF.sub.4 system described above. However, these direct substitutions were not glass forming compositions so he varied the molar ratios of the four elements until he found some areas of glass formation. He later added lead in place of some of the barium and zinc in an effort to increase the glass forming character.
In the fall of 1995, several companies expressed interest in the lead free glass compositions reported by Zhou. This led to an effort, through the Iowa State University Research Foundation (ISURF) to reproduce some of these glasses. Attempts at reproducing his work failed as the compositions were found to have a severe tendency to crystallize at a thickness as small as 1 mm. It was later found that Zhou had used porcelain crucibles to melt some of his compositions. It is assumed that the alumina and silica oxides from the crucibles aided in glass formation, and may have actually produced an oxy-fluoride glass, as fluoride will readily attack any oxides present at elevated temperatures. During attempts reported herein to reproduce his work, platinum crucible, known not to contaminate fluoride glasses, were used.
Even though the lead free compositions were not good glass formers, there was evidence that maybe some of the compositions could be modified and/or stabilized to produce a family of good glass forming compositions. This evidence came from the fact that when a melt was quenched between two plates at room temperature, rather than at an elevated casting temperature, thin sheets of glass were formed.
The purpose of the present invention is not only to find a high density glass free of lead and radioactive elements, but also to find a stronger glass forming system since most applications would require a glass with a thickness greater than 3 mm.